This invention relates to a method of optimizing the collection efficiency of a dry-process electrostatic precipitator which is preceded by an evaporative cooler, wherein liquid is sprayed into the evaporative cooler at a rate which is controlled so as to maintain a desired temperature.
Electrostatic precipitators are often preceded by an evaporative cooler in order to improve the collection efficiency of a precipitator. The evaporative coolers reduce the gas temperature and exert a desirable influence on the collection of the dust entrained in the gas stream. Besides, the coolers effect a certain preliminary collection of dust because dust particles agglomerate with the sprayed liquid to form larger particles, which are separated by gravity at points of reversal or where the gas velocity is decreased. The dust resistivity is a parameter which influences the collection of the dust in the electrostatic precipitator and often exceeds 10.sup.11 ohm-cm. In that case a collection of dust from a gas by means of an electrostatic precipitator cannot be effected with a reasonable expenditure unless the gas temperature is lowered in order to reduce the dust resistivity below that value (see Z. Technische Mitteilungen, 71, (1978), pages 123 to 131, particularly FIG. 17 on page 127).
The dust resistivity will be lower and the collection efficiency will be higher as the gas temperature decreases. But owing to the risk of corrosion the gas temperature must not decrease below the dew point of acid. For this reason the temperature of a gas stream from which dust is to be collected in an electrostatic precipitator cannot be reduced by means of an evaporative cooler below a lower limit, which depends on various variables and which may fluctuate more or less during operation.
In the design of a dust-collecting plant comprising an evaporative cooler and an electrostatic precipitator it is not sufficient, for the reasons explained above, to measure as exactly as possible the anticipated dew point of acid and to take that anticipated dew point properly into account. For optimizing the collection efficiency and for adjusting the plant in dependence on a varying composition of the gas, the operation of the evaporative cooler must continually be adapted to the actual conditions.
This involves such a great difficulty, that it has not been possible before to reliably and continuously measure the dew point of acid. Because the known devices for measuring the dew point of acid will inevitably be soiled, they are liable to malfunction and require a careful servicing. They must be cleaned virtually after every measurement in order to preclude measuring errors and to ensure a satisfactory function. For this reason automatic measuring devices are highly expensive and complicated and can be used, at best, for an intermittent measurement of the dew point of acid. Such an intermittent measurement cannot be used to control the evaporative cooler with the aim of optimizing the collection efficiency of the electrostatic precipitator. The measured values become available only with long intervals of time and a control system utilizing such measured values could not respond to changes in operating conditions so quickly that corrosion will be reliably avoided. Whereas that risk might be avoided in that an adequate margin of safety from the dew point of acid is maintained in operation, this would prevent an optimizing of the collection efficiency. On the other hand, there may be no changes in the dew point of acid for long periods of operation so that a continuous use of the complicated measuring devices is not justified.